Root-form dental implants have been used with success rates of over 90% to replace teeth lost to various disease processes. The initial clinical use was to replace 12 to 14 teeth per arch on 6 to 8 implants. This is in the fully edentulous situation, and is rigidly fixed in place by retaining screws. In recent years many more applications have evolved in the partially edentulous patient. These include fixed bridges on as few as two implants, but lacking the tripod effect provided by three or more implants. These also include single tooth restorations, that is one tooth on one implant. The long-term success rates of these more recent types of restorations are not known.
Current implant restorations typically conduct occlusal forces through porcelain, gold alloy, titanium, then to bone. All of these structures are rigidly fixed together with screws.
Implants generally consist of three components. The fixture, fabricated in titanium, is surgically placed into the jawbone. A process called osseointegration occurs, in which bone forms a direct contact with the titanium fixture. Any fibrous tissue or other tissue type is considered to be a lack of osseointegration and constitutes a clinical failure of the implant fixture. In fully edentulous cases, failure of one or two of, say, eight implant fixtures does not cause the entire prosthesis to fail. However, in partially edentulous and single tooth applications, failure of even one implant fixture normally results in failure of the entire case. A new fixture can subsequently be placed, but this considerably delays the case and there is no guarantee that the new fixture will work either. It should be noted that approximately 50% of failures occur before loading with occlusal forces via a prosthesis, and most of the remaining failures occur within one year of each loading.
The second implant component is the abutment, which is a cylindrical titanium construct with a large titanium abutment retaining screw. The abutment is precisely machined to fit the fixture; any abutment should fit any fixture with the same degree of exactness. The abutment passes from the distal surface of the fixture through the gingival tissue. Various abutment types are available. Some pass entirely through the gingiva so that their distal surface is visible in the mouth. Others end subgingivally for better aesthetics. Aesthetics depends on the contour of the restoration and the exact position of the distal surface of the abutment.
The third implant component is the prosthesis. This is custom-cast gold alloy with a veneer of porcelain, and forms the actual "teeth". Casting is accomplished by either:
1) A plastic cylinder, fitting to the distal surface of the abutment, which is cast in the lost wax technique, or
2) A machined gold alloy to which additional gold can be cast to form the tooth.
Porcelain is added after the metal frame is cast. The prosthesis is rigidly held in place by one or more prosthesis retaining screws. These are smaller than the abutment retaining screws and are made from gold, not titanium. Both screws are tightened with 5 to 10 Newtons of torque; this process causes the fixture-abutment-prosthesis assembly to be held rigidly together.
Restorations on implants can be esthetic enough that they resemble natural teeth in color, contour and relationship to the gingiva. Titanium and gold can be hidden in the subgingival area, the sulcus. In these restorations, contoured and shaded porcelain can be carried below the gingival crest, into the subgingival area. For technical reasons, this porcelain must end 1 mm above the proximal surface of the abutment. Consider the example of a 4 mm sulcus (the distance from the crest of the gingiva to the distal surface of the fixture). If the desired result is a porcelain, natural tooth growing out of the gingiva, the abutment can be no more than 2 mm in height. This leaves 1 mm for the minimum metal showing on the prosthesis and 1 mm of subgingival porcelain as a reserve in the case of recession or shrinkage of the gingiva.
Thus, many of the drawbacks of earlier implant restorations, particularly aesthetics deficits, have been overcome. However, concern has arisen over the rigidity of the complete implant system. The system possesses a very high modulus of elasticity, which means that it has very little dimensional change per unit of force applied. Occlusal forces are transmitted in a direct manner through the entire system into the bone. Some concerns are:
1) The natural tooth possesses a periodontal ligament, which dampens occlusal forces. Lacking this dampening may cause failure (fracture or bending) of implant components. Indeed, the prosthesis retaining screw is gold and smaller than the titanium abutment retaining screw. This provides a "weak link"; the easiest component to break is also the easiest to remove and replace. Thus, implant design anticipates some percentage of component failure.
2) Fixed bridges from natural teeth to implants have historically had poor success rates. The implants are so rigid that they support the restoration while the teeth drift out of function. The resulting torque may cause restoration failure or implant component fracture.
3) The different pattern of stress induced by the more direct force transmission may cause failure of the bone-titanium interface (osseointegration). Long-term (&gt;10 year) studies on this aspect of the partially edentulous and single tooth situations do not exist.
4) Extreme accuracy of fit is required between the prothesis and the superior surface of the abutment. Any minor imperfections in impression making and prosthesis fabrication become exceedingly damaging due to the lack of elasticity. The strain induced by these inaccuracies may cause failure of the bone-titanium interface.
Various intramobile elements have been designed to simulate the function of the healthy periodontal ligament as occlusal function is transmitted through the restoration. Such function in humans varies on a range of 100-2,440 Newtons in an axial direction, and is on the order of 20 Newtons in a lateral direction. A healthy periodontal ligament may depress and rebound 0.1 mm. An implant prosthesis, being rigid, will depress several orders of magnitude less. The goal of an intramobile element is to provide an implant system with a degree of movement under function which is similar to that of a natural tooth.